The England defender sent the Foxes away contingent into raptures with a 25-yard effort which beat Saints goalkeeper Alex McCarthy in the 92nd minute of the Premier League game.
"Tough game," Puel told BBC Sport. "Southampton played very well in the first half with intensity and it was difficult to find space. After, we found more space in the second half and i think the result was deserved.
"We dominated, we had a lot of chances and we had two or three chances more to score. First half it was good defending and I think we deserved this because we could've scored before the second half.
"Harry is a very good player for us. It is fantastic for him to score, for him and the team, it was a very good game. We have to keep this mentality to the end when we come back in the game. it's fantastic."
Maguire found himself linked with Manchester United over the summer thanks to his impressive displays for club and country over the past few months.